ugar and spice and everything nice, what are little diatoms made of?
Diatoms are made of soft organic materials encased within a hard opaline shell. The shell, which is called the frustule, is a transparent glassy structure that comes in many fascinating and beautiful shapes. In fact, the intricate architecture of the frustule is what scientists use to distinguish one diatom species from another.

The composition of the frustule is very similar to the gemstone opal. They both contain the elements silicon and oxygen along with water molecules. Diatoms grow best where silica, as well as sunlight and other nutrients, are plentiful.

There are two parts to the diatom's frustule. Each part is a shallow, half-cylinder called a valve. Since one of the valves, the epitheca, is slightly smaller than the other valve, the hypotheca, the two parts fit together like a pill box, encasing the protoplasm inside. However, due to their extreme ornamentation, it is sometimes impossible to distinguish the epitheca from the hypotheca from the fixed view of a single photograph. Can you point out the hypotheca on the specimen shown below?

• Silica The term silica is used to refer to a group of minerals that contain the elements silicon and oxygen. Opal is considered to be a type of silica. Thus, you might read or hear that diatom shells are composed of either opal or silica, and that their shells are opaline or siliceous.
• Silicon The mineral matter silica and the element silicon are not synonymous. The element silicon is the silvery material used to make computer chips. It is also the second most abundant element in the earth's crust.
• Silicone The term silicone refers to an entirely different chemical compound, a polymer that also contains the element silicon. Rubbery silicone is commonly used in caulking for bathtubs and sinks.